Field of the Invention
The present invention relates to a coating material and also a process for producing a corrosion protection layer and/or adhesion promoter layer.
Discussion of Background Information
In metallic components which are exposed to high temperatures and/or corrosive media, for example components of flow machines in the form of stationary gas turbines or aircraft engines, the metallic surfaces have to be protected from oxidative and/or corrosive attack. Many different layer systems are known for this purpose in the prior art.
Use is made of, inter alia, phosphate layers in the case of which sparingly soluble phosphates are precipitated on the metal surface to be protected from a solution containing phosphate ions so as to form a protective layer which is firmly bonded to the metallic material to be protected and can therefore also serve as adhesion promoter layer for additional protective layers arranged on the phosphate layer.
In this type of coating of the metallic surface, metal cations from the phosphate solution can additionally contribute to the layer structure and provide a corrosion protection effect. For example, aluminum ions or aluminum particles can be provided in the phosphate solution in order to achieve accumulation of aluminum in the phosphate-based corrosion protection layer and/or adhesion promoter layer to be formed, with the aluminum being able to contribute to corrosion protection by formation of a slowly growing aluminum oxide layer. An example of such a coating material is given in the U.S. Pat. No. 3,248,251.
Such a coating material usually additionally contains chromates in order to make stabilization of the phosphate solution possible. Otherwise, particularly in the presence of metallic particles such as aluminum powder in the phosphate solution, reactions of the phosphoric acid with the metal powder or aluminum powder can occur. However, chromates and in particular chromium(VI) ions are toxic and hazardous to health, so that efforts have been made to remove chromates from such coating materials. An example is given U.S. Pat. No. 6,368,394 B1 in which a coating composition which comprises water, phosphate ions, borate ions and aluminum ions and has a pH in the range from 1.4 to 2.2 is proposed.
However, there continues to be a need to provide a coating material for a process for producing a phosphate-based corrosion protection layer and/or adhesion promoter layer and also a corresponding process in which, firstly, the stability of the coating material for the phosphate-based coating is ensured and, secondly, effective layer formation on the metallic component to be protected is also achieved.